Neutron logging is a common measurement used in both wireline logging and logging while drilling operations as an indication of formation porosity. In neutron logging operations, a neutron source emits high energy (“fast”) neutrons into the formation. Americium-241/Beryllium (AmBe) and Californium-252 are common chemical neutron sources. A flux of fast neutrons can alternatively be generated by an electrical source such as a d-T or d-D generator. The fast neutrons are slowed by the surrounding formation (particularly via collisions with hydrogen nuclei present in the formation and the borehole and eventually captured). The capture of a neutron may result in the emission of one or more prompt gamma rays. While, neutron logging tools can be configured to detect the capture gamma rays, epithermal and/or thermal neutrons are most commonly detected using one or more thermal or epithermal neutron detectors. In most applications these detectors are 3He proportional counters.
FIG. 1 depicts a common configuration of a prior art neutron logging tool 10. In the configuration depicted, a sealed chemical source 12 is deployed in a tool body in close proximity with (e.g., within a few feet) and longitudinally spaced from first and second longitudinally spaced neutron detectors 14 and 16 (commonly referred to in the art as near and far detectors).
During a subterranean logging operation the ratio of the neutron count rates at the near and far detectors (the near to far ratio) is commonly taken to be indicative of liquid-filled formation porosity and/or the hydrogen concentration (hydrogen index) of the formation. In formations having a high concentration of hydrogen, the emitted neutrons are slowed down more efficiently and captured in closer proximity to the source. As a result, a relatively small number of neutrons are detected at the far detector, resulting in a relatively high near to far detection ratio. This high ratio is commonly interpreted as being indicative of high porosity (since the hydrocarbons and/or water tend to occupy pore space in the formation). In formations having a low concentration of hydrogen, the emitted neutrons tend to travel farther. This results in a higher count rate at both detectors and a lower near to far detection ratio. A low ratio is thus commonly interpreted as being indicative of low porosity.
Those of ordinary skill in the art will readily appreciate that the above described mechanism is highly simplified and that in practice the interpretation of neutron logs can be complicated by numerous factors. Despite the fact that neutron logging techniques have been in commercial use for over 50 years, the interpretation of neutron logs remains challenging (and is considered by some to be an art). For example, Ellis et al states that there are numerous “mysterious effects that must be dealt with when using neutron porosity logs” (emphasis added) (Ellis, Case, and Chiaramonte, Petrophysics, 2003, 44(6), p. 383). There is clearly a need in the art for improved tools and methods for making and interpreting neutron logging measurements.